In the field of surveillance and security of industrial sites or lengths of roads or tunnels, the speed of fire detection constitutes a predominant security factor. In particular, it is necessary to be able to detect the start of a fire as quickly as possible in order to be able to fight it efficiently and to take measures to limit the extent of the blaze. For cost reasons, it is however generally impossible to employ human surveillance continuously. Automatic surveillance and detection systems are thus highly desirable.
Different systems have already been proposed or commercialized in order to detect fires or smoke.
Most of the currently used systems make use of punctual smoke sensors that must wait until the smoke propagates up to them to have a chance of detecting it. These sensors are unusable outside (refineries, storage of containers, etc.), on large premises in which the smoke disperses and takes a long time to reach the sensor (hangars, nuclear power station, etc.) or on premises with strong air draughts (tunnels, strongly ventilated rooms, etc.). The sensors must be sufficiently near and wired; the cost of wiring a large number of sensors can however prove prohibitive. These solutions are thus poorly suited for the surveillance of large volumes or ranges.
Other known systems are based either on measuring the temperature increase in the room, or on measuring the received quantity of UV or infrared radiation.
Systems using the temperature-increase principle are relatively slow (temperature lag) and do not function reliably outside or on large premises. Systems based on measuring UV radiation function in any environment but quickly loose their efficiency when the sensor is soiled, without this being detectable.
Systems based on measuring infrared radiation function in any environment but generate false alarms when they are in the presence of a hot object or when they are exposed to solar radiation.
More recently, it has been suggested to detect fires with the aid of methods based on image analysis. Many potentially dangerous sites are already equipped with surveillance cameras connected to a central fire alarm system and used for example to detect breaking-ins or accidents. Use of these surveillance systems for also detecting fires makes it possible to save the costs of installing and connecting a separate system of sensors. Automatic image analysis solutions, using already installed video cameras and software for processing the video signals supplied by the cameras, have also been suggested.
Smoke detection by image analysis has the following advantages over the solutions using punctual sensors:                The camera can detect smoke and flames at a distance, before they even reach the sensor, therefore such a system is capable of avoiding the deficiencies of the traditional systems outside or on large premises.        The images taken by the camera can not only be processed, but also used for visualizing the incident by an operator. This is useful in order to remove any doubts in the case of false alarms: visualizing the image or image sequence by a human allows many unnecessary trips to be avoided.        The images taken make it possible also to give a better idea of the magnitude of the blaze as well as of the type of fire. It is thus possible to immediately prepare the correct intervention material and to thus save precious minutes.        A soiling of the sensor (camera) is visible on the image and, according to the invention, can even be detected automatically, contrary to the UV radiation sensors that lose their efficiency without this being detectable.        Malfunction or sabotage of the camera is detectable automatically.        The camera used for detecting fires is usable simultaneously for classical surveillance applications, which allows the wiring to be simplified.        
Systems for detecting fires by analyzing video images have already been described in the prior art. WO00/23959 describes a system for detecting smoke, consisting of a video camera equipment, a unit for digitizing video signals and a unit for processing digital data. The smoke is detected by image processing algorithms based on comparing the pixels between successive images. The comparison methods used aim for example to detect if an important change has occurred between an image and a reference image that could indicate the appearing of smoke but also of any other object within the filmed visual field. Another algorithm detects the convergence of color of several pixels towards an average value, capable of indicating a drop in contrast caused by smoke. Such a convergence can also indicate a change in the lighting conditions. A third algorithm measures the changes in the sharpness of the transition zones, affected by smoke but also by optic characteristics that are modified for example during zooms or changes of aperture. These methods are uniquely adapted to detecting smoke but not flames emitting little or no smoke. The algorithms employed are complex and require considerable processing power.
WO97/16926 describes a method for detecting changes in an image sequence in order to detect events. The method of detection is based on taking a reference image that contains the background information of the recorded scene. The appearing of new objects is detected by thresholding and pixel grouping methods. The algorithms employed are poorly suited to distinguish between the appearing of smoke or of any other object in the filmed visual field.
EP0818766 describes a system for detecting forest fires by processing animated images. To detect the fire, a smoke detection algorithm is used. This document describes a method for detecting temporal variations of the pixels' intensity at low frequency (between 0.3 and 0.1 Hz). The system is thus fairly slow to react since many cycles of several tenths of seconds are necessary to detect a de-correlation that could indicate the presence of smoke.
FR-A-2696939 describes a system for automatically detecting forest fires by image processing. Processing algorithms are based on the detection and analysis of the movements of volutes and clouds of smoke; they are however poorly adapted for detecting flames or smoke that develop in unusual ways, for example under the effect of wind or of a ventilation.
The existing systems for detecting fire by analysis of video images are well suited for detecting particular types of fire in well-defined environments. A firm wishing to specialize in the surveillance of fires in different sites must however acquire and become acquainted with different software programs; there is at the present time no solution sufficiently robust and polyvalent that allows the detection of very different fires by means of the same software.